The present invention relates to semiconductor device structures and, more particularly, to FET device structures formed on the same substrate, and to methods for their manufacture.
In CMOS technologies, nFET and pFET devices are optimized to achieve required CMOS performance. The dopant species used for nFET and pFET devices have different physical properties, accordingly. For example, diffusivity and maximum active concentration vary significantly for different dopant species. In conventional CMOS technologies, nFET and pFET share the same spacer process for the source/drain implant. The common spacer process forces the source-drain implant to have the same off-set distance from the edge of the gate electrode for both nFET and pFET. In order to optimize CMOS performance, the spacers typically are of one width and are designed to trade-off the performance between nFET and pFET. For example, if Arsenic and Boron are used as the source/drain dopants for nFET and pFET, respectively, it is known that a narrower spacer is better for nFET, because the shorter distance between source and drain results in an increased drive current, but a much wider spacer is needed for the pFET, because the increased diffusivity of Boron compared to Arsenic results in a degraded short channel effect control for the pFET. In this case, the pFET is a limiting factor because good short channel effect control is a strict requirement for a CMOS technology. Thus, the maximum width of all spacers is optimized for pFET, trading-off the nFET performance. See, for example, U.S. Pat. No. 5,547,894 (Mandelman et al., issued Aug. 20, 1996, entitled “CMOS Processing with Low and High-Current FETs”); U.S. Pat. No. 4,729,006 (Dally et al., issued Mar. 1, 1998, entitled “Sidewall Spacers for CMOS Circuit Stress Relief/Isolation and Method for Making”); and U.S. Pat. No. 4,648,937 (Ogura et al., issued Mar. 10, 1987, entitled “Method of Preventing Asymmetric Etching of Lines in Sub-Micrometer Range Sidewall Images Transfer”); which are all incorporated by reference herein in their entireties.
It is a problem, therefore, to optimize spacer width and FET performance for both the nFET and the pFET on the same substrate.
One known method of improving drive current without degrading the short channel effect control is to improve charge carrier mobility. It is known that for devices oriented so that the current flows along the 110 direction, which is the industry standard, stress applied along the direct of current flow can influence charge carrier mobility. Specifically, compressive stress applied along the direction of current flow (longitudinal direction) increases hole mobility while tensile stress applied in the longitudinal direction increases electron mobility. Although longitudinal compressive stress increases hole mobility, the same longitudinal compressive stress degrades electron mobility. A similar phenomenon is true for longitudinal tensile stress. Tensile stress applied in the longitudinal direction degrades hole mobility.
It is also known that hole mobility is not degraded when tensile stress is applied along the longitudinal direction if the device is oriented so that the current flows along the 100 direction. See, for example, U.S. patent application No. 2002/0063292 A1, entitled “CMOS Fabrication Process Utilizing Special Transistor Orientation” by Armstrong et al., filed Nov. 29, 2000, published May 30, 2002, which is hereby incorporated in its entirety by reference. However, this prior art method requires major design modifications to orient the devices along different directions. In addition, this method apparently is only directed at improving mobility for nFETs and has no provisions for improving mobility for pFETs. Because performance is improved for only one device and overall circuit performance depends on the performance of both devices, the overall benefit will be limited. While it has been possible to improve mobility for one device, it has been difficult to improve mobility for both devices at the same time.
Therefore, a method of improving electron and hole mobility on the same substrate would be beneficial. It would be even more beneficial if the method of improving electron and hole mobility simultaneously provided a means for reducing (preferably, eliminating) the trade-off problem between short channel effect control for the pFET and drive current increase for the nFET.
The present invention solves these problems by using a dual-spacer width in combination with film stress optimization for the spacer and etch stop films. This combination permits optimizing nFET and pFET device performance independently while simultaneously improving charge carrier mobility for both electrons and holes on the same substrate.
It is a principal object of the present invention to optimize performances of two different MOS devices having a common semiconductor substrate.
It is an additional object of the present invention to optimize independently the performances of an nFET device and a pFET device formed on one substrate.
It is a further object of the present invention to increase the drive current performance of an nFET device while decreasing short channel effects in a pFET.
It is yet another object of this invention to increase charge carrier mobility for electrons and holes on the same substrate.